1. Field of the Invention
The invention relates generally to medical imaging technology and more particularly to breast cancer diagnosis using the measurement and display of tissue stiffness properties.
2. Description of the Prior Art
X-ray mammography is the primary method for early detection of breast cancers. According to the reports of US Food and Drug Administration, mammography can diagnose 85 to 90 percent of the breast cancers in women over the age of 50, and can detect a lump up to two years before it can be sensed by manual palpation. While it is easier to detect malignancies as age increases and the breast tissue becomes more fatty, mammography fails to detect small cancers in dense breasts. Further, mammography may not be specific in terms of tumor benignity and malignancy, especially when the breast tissue is radiodense. A significant number of suspicious masses referred by mammography for surgical breast biopsy are, in fact, benign. False-positive mammograms induce anxiety, distress and emotional disruption in patients.
It has been well recognized that the tissue stiffness plays an important role in diagnosis of breast cancers, as tumors are stiffer than the surrounding breast tissues, and malignant tumors are much stiffer than benign ones. In other words, in vivo identification of the elastic moduli of normal and abnormal breast tissues, which describe the stiffness, should improve the accuracy for breast-cancer diagnosis.
There have been elastography studies based on ultrasound or MRI breast imaging. Developed in 1990s by Ophir (Ophir, J. et al., 1999, “Elastography: ultrasonic estimation and imaging of the elastic properties of tissues,” Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine, vol. 213, no. 3, pp. 203-233), ultrasound elastography (USE) was the first modulus-imaging modality. Although capable of providing new information on detecting pathological tumors, USE suffers from limitations in the detectable stiffness range which is imposed by the minimum resolvable wavelength. Most USE elastograms are referred to as the strain imaging, which may not always provide useful information on the locations and characterizations of heterogeneous lesions. For example, the phenomenon known as “butterfly wings” (Ophir et al. 1999) is frequently observed in the USE strain elastograms, which may be misleading with respect to tumor detection.
Magnetic resonance elastography (MRE) was developed more recently as the second-generation elastography modality. MRE is capable of producing adequate spatial and contrast resolutions. It is, however, a high cost MR imaging procedure, making it less practical for many patients. In addition, the penetration depth of shear waves within organic tissue is limited to only a few centimeters. Due to a large frequency-dependent attenuation, only low-frequency waves of about 50-100 Hz are usable. This limits the spatial resolution and the achievable detectability of small lesions.
Thus, new approaches of elastography are still needed to achieve quick, low-cost, and low-dose screening and diagnosis of breast cancers.